Today's electronic systems typically require a relatively high amount of current from one or more highly regulated power supplies. A typical computer system will draw 50–60 Amperes (A) (laptop) or 80–120 As (desktop/server) from a 1.2 to 1.3 volt (v) power supply. In the past, linear power supplies have been used, but a significant amount of power is typically dissipated across the pass element. As a result, pulse-width modulated (PWM) switching power supplies were introduced to provide power in a more efficient manner. Switching power supplies dissipate less power than linear power supplies because the drive transistors operate in either full-on or full-off mode. Specifically, in the full-on mode, the voltage drop across a drive transistor is minimal, and in the full-off mode, no current flows through the drive transistor. Therefore, in the full-on mode, there is low power dissipation in the drive transistor even at a relatively high current.
To provide relatively high output currents, conventional PWM power supplies include one or more main channels or phases. Each phase is driven by a PWM controller chip and includes a driver, a pair of output drive transistors arranged in a push/pull configuration, and a filter inductor, which is coupled to a filter capacitor which is common to all phases. Because the drive transistors provide relatively high current, they are relatively high-power devices that, in turn, have relatively slow on/off times. Therefore, it is desirable to drive a main phase with a relatively long pulse width so that the high-power drive transistors have sufficient time to turn on and operate in the full-on mode. That is, for the best efficiency, the pulse width should be much greater—for example, at least ten times greater—than the longer of the drive transistors' on and off times.
Of course, without proper filtering, longer on times at relatively high currents will typically cause the drive transistors to rather quickly, overcharge the filter capacitor and, thus, drive the output voltage above the regulated range. One way to prevent this over-voltage situation would be to drive the transistors using relatively short pulse widths. But as stated above, because these transistors have relatively slow on/off times, such operation would be very inefficient.
Consequently, a filter inductor is typically placed between the drive transistors and the filter capacitor. From the standard inductance equation V=LdI/dT, one can see that dI/dT=V/L, where V is the voltage across an inductor, L is the inductance, and I is the current through the inductor. Therefore, one selects the size of the inductor such that dI/dT is small enough to allow the relatively long pulse widths to drive the transistors without overshooting the regulated voltage. Although this allows the drive transistors to operate efficiently and to provide large amounts of power, the response time of each main phase to transients (caused by sudden demands for either an increase or a decrease in power from the power supply) is relatively slow. Therefore, such transients can cause the power supply voltage to temporarily go out of the regulated range, i.e., cause the PWM power supply to temporarily lose regulation and allow the supply voltage to spike. Unfortunately, if such a transient is large enough or long enough, it may cause a malfunction such as corruption of data stored in a memory.
To reduce the size of the drive transistors and filter inductors, and thus to allow an increase in the power supply's transient response, a typical PWM power supply includes multiple main phases that, in a steady state condition, operate in an alternating switching pattern such that at least one main phase is always on. This allows the different main phases to share the power supplying duties to the load. Although this sharing requires more circuitry, it allows faster drive transistors and smaller filter inductors to be used. But unfortunately, even a multiple-main-phase PWM power supply is often unable to prevent relatively fast transients from occurring on the regulated power supply voltage.
Another way to reduce the magnitude of occurrences of power-supply transients is to use a larger filter capacitor that has a relatively low equivalent series resistance (ESR) and an acceptable high-frequency response. However, such a filter capacitor take is often relatively large and expensive.
Alternatively, one can add a linear regulator to a PWM regulator to reduce the magnitude of occurrences of undesirable transients. The linear regulator, which is less efficient but has a faster response time than the PWM regulator, is activated only when a transient occurs to provide the fast correction response. This solution is described in detail in U.S. Pat. No. 5,926,384 to Jochum et al. But problems with this solution include the complexity and space requirements of adding a linear regulator, and also the inefficiency of dissipating power across the linear regulator's pass element.